ERPs reveal individual differences in morphosyntactic processing.

We investigated individual differences in the neural substrates of morphosyntactic processing among monolingual English speakers using event-related potentials (ERPs). Although grand-mean analysis showed a biphasic LAN-P600 pattern to grammatical violations, analysis of individuals׳ ERP responses showed that brain responses varied systematically along a continuum between negativity- and positivity-dominant ERP responses across individuals. Moreover, the left hemisphere topography of the negativity resulted from component overlap between a centro-parietal N400 in some individuals and a right hemisphere-dominant P600 in others. Our results show that biphasic ERP waveforms do not always reflect separable processing stages within individuals, and moreover, that the LAN can be a variant of the N400. These results show that there are multiple neurocognitive routes to successful grammatical comprehension in language users across the proficiency spectrum. Our results underscore that understanding and quantifying individual differences can provide an important source of evidence about language processing in the general population.